With the recent reduction in size and increase in capacity of a power semiconductor module, a power semiconductor chip in a power semiconductor module (e.g., an insulated gate bipolar transistor (IGBT)) is therefore operated at high current density. The crucial problem with such power semiconductor module, therefore, is how to dissipate heat generated therein.
Specifically, while an upper limit of the guaranteed temperature is set for a joint temperature Tj of a semiconductor chip in a power semiconductor device such as an IGBT, in a one-sided cooling system in which a semiconductor chip is mounted on a heat dissipating base (copper base plate) with a patterned insulating substrate therebetween, heat dissipation from the upper surface of the semiconductor chip is hardly expected because the upper surface of the semiconductor chip is sealed with sealing resin filling the package. As the heat density increases due to the reduction in size and increase in current of the semiconductor chip, the conventional wiring structure in which an aluminum wire is used as a wiring lead to be connected to an upper surface electrode of the chip for bonding cannot easily reduce the joint temperature of the chip to the upper limit guaranteed temperature or lower. Moreover, combined with Joule heating from the aluminum wire, wire fusing may occur, which brings concern that the power cycle resistance of the semiconductor chip becomes low.
As the means to improve heat dissipation of the upper surface of the semiconductor chip, a lead frame, for example, can be used in place of the aluminum wire. In this case, a lower lead frame is soldered and attached to an upper surface main electrode of the semiconductor chip, and an upper lead frame is disposed on this attached lower lead frame in such a manner as to be superimposed on the ends thereof. A module structure is known in which an upper lead frame and a lower lead frame are laser-welded and attached to each other and used as a heat transfer path to release heat of the semiconductor chip from the upper surface thereof.
FIGS. 11 to 13 are configuration diagrams of a conventional power semiconductor module. FIG. 11 is a plan view showing substantial parts of the conventional power semiconductor module, FIG. 12 is an enlarged view of a section A shown in FIG. 11, and FIG. 13 is a schematic cross-sectional view of substantial parts, taken from line X-X of FIG. 11. FIG. 13 also shows a laser beam.
A power semiconductor module 500 is configured by a heat dissipating base 51, a patterned insulating substrate 56 attached onto the heat dissipating base 51, with a solder 52 therebetween, and a semiconductor chip 58 attached onto a conductive pattern 55 of the patterned insulating substrate 56, with a solder 57 therebetween. In the patterned insulating substrate 56, a conductive film 53 is formed on a rear surface of an insulating substrate 54 and the conductive pattern 55 on a front surface of the same.
The power semiconductor module 500 is also configured by a lower lead frame 60 attached to an upper surface electrode of the semiconductor chip 58 with a solder 59 therebetween, an upper lead frame 61 laser-welded and attached to the lower lead frame 60, a resin case 62 that is attached to the heat dissipating base 51 by a silicone adhesive 62a and fixed, in a penetrating manner, to the lower lead frame 60 serving as an external terminal, and gel, not shown, which fills the resin case 62. The lead frames 60 and 61 are disposed randomly without specifying a direction D of rolling traces 69 (a rolling direction) formed on the lead frames 60 and 61.
The upper lead frame 61 and the lower lead frame 60 are laser-welded and attached to each other as described above, and the attached portions therebetween are electrically connected to configure current paths.
When laser-welding the lower lead frame 60 and the upper lead frame 61 to each other, a laser beam 64 is radiated from a laser emitting unit 63 onto the upper lead frame 61, to melt and solidify the upper and lower lead frames 60 and 61.
The laser beam 64 is radiated at an angle θ of approximately 10° to 15° with respect to a vertical line 66 on the irradiated surface of the upper lead frame 61.
Further, attached portions 65 (laser-welded sections) to be obtained as a result of the radiation of the laser beam 64 are specified by a camera and a light source, not shown, which are installed in the laser emitting unit 63 and a monitor, also not shown, which is installed outside the laser emitting unit 63. Visible light is emitted from the light source. Resultant reflected light from the irradiated surface is captured by the camera installed in the laser emitting unit 63, and the camera transmits a signal corresponding to the reflected light, to the monitor. Through the image displayed on the monitor, the irradiation section can be specified. Because the optical axis of this visible light coincides with an optical axis 67 of the laser beam, the visible light, too, is radiated at an angle θ of approximately 10° to 15° with respect to the vertical line 66 on the irradiated surface.
The number of joining points and joining areas corresponding to the attached portions 65 between the upper lead frame 61 and the lower lead frame 60 are determined in consideration of the conditions under which the power semiconductor module 500 is used (heat generation due to electrification, joint strength in relation to the reliability).
Extra points between the upper lead frame 61 and the lower lead frame 60 are laser-welded in consideration of inconsistent laser welding occurring therebetween.
Patent Documents 1 and 2 disclose the technologies for laser-welding a plurality of lead frames.
Patent Document 3 describes a method of manufacturing a semiconductor device in which a lead frame is roughened for the purpose of reducing the laser welding inconsistency.    Patent Document 1: Japanese Patent Application Publication No. 2008-98585    Patent Document 2: Japanese Patent Application Publication No. 2008-42039    Patent Document 3: Japanese Patent Application Publication No. 2008-28286
FIGS. 14(a)-14(c) show a method of forming a lead frame and FIGS. 14(a) to 14(c) are process diagrams showing respective steps in sequence. First of all, a thick copper plate 72a is rolled with a roll 71 into a copper plate 72 of a predetermined thickness, as shown in FIG. 14(a). Next, a desired lead frame 61 is punched out of the copper plate 72 of the predetermined thickness, as shown in FIG. 14(b) (the upper lead frame is shown in this diagram). Thereafter, the lead frame 61 is removed from the copper plate 72, and a predetermined section of the lead frame 61 is bent (not shown), as shown in FIG. 14(c). As a result of rolling the copper plate 72a, stripe-shaped rolling traces 69 are formed on the surface of the lead frame 61. Because these rolling traces 69 are formed in the rolling direction D, the direction D of the formed rolling traces 69 is same as the direction D in which the rolling is performed (the rolling direction).
As shown in FIG. 12, the direction D of the rolling traces 69 of the upper lead frame 61 lies in a random direction (diagram shows the rolling traces 69 formed in perpendicular and parallel directions) when the upper lead frame 61 is disposed. When radiating visible light to the upper lead frame 61 in order to detect the sections to be laser-welded, the fact that the direction D of the rolling traces 69 of the upper lead frame 61 lies in a random direction intensifies the fluctuations of the intensity of the visible light reflecting off of the irradiated surface. As a result, the visibility of the camera drops, erroneously detecting the sections to be welded.
An Ni-plated layer having a thickness of approximately 10 μm, for example, is formed on the surface of the lead frame 61 (Cu) in order to increase its absorption rate of the laser beam 64.
None of Patent Documents 1 to 3 mentions that the rolling traces of the plurality of lead frames are arranged in the same direction (the rolling direction) to perform laser welding.